The present invention generally relates to a high frequency, high performance telecommunication cable for commercial building applications and, in particular, relates to one such high frequency telecommunication cable including a plurality of twisted pair conductors disposed about a plurality of soft support means.
Historically, early telecommunication cable designs have suffered from the dynamic, inductive effects of parallel and adjacent conductors. Also generally known as "crosstalk", this problem becomes even more severe at high frequencies or high data rates and over long distances. Thus, crosstalk effectively limits the frequency range, bit rate, cable length, signal to noise ratio as well as the number of conductor pairs which can be used within a single cable for signal transmission. Further, the higher the number of potentially "energized" conductors or pairs there are in the cable, the more potential exists for crosstalk interference. Crosstalk can be even more pronounced in bi-directional transmission cables. Generally known as "near end crosstalk", the effect is particularly noticeable at either end of the cable where signals returning from the opposite end are weak and easily masked by interference. It quickly became known in the art that crosstalk could be better controlled by separating parallel and adjacent transmission lines or by transposing the signals along the cable to minimize the proximity of any two signals. For example, U.S. Pat. No. 445,234 issued to Reilly on Jan. 27, 1891, discloses a single conductor arrangement where signals are transposed at various locations along the length of cable so that no two conductors would occupy the same relative positions. Although physically separating conductors sufficiently to limit crosstalk in a single, compact cable proved difficult, several such designs emerged. For example, U.S. Pat. No. 473,267, issued to Sawyer on Apr. 19, 1892, describes a technique for braiding single conductors to maintain spacing among adjacent conductors and thereby reduce capacitance and reduce strain. Similarly, U.S. Pat. No. 1,305,247, issued to Beaver, et al. on Jun. 3, 1919, describes the use of a rubber insulator between two conductors for adding elasticity without damaging the conductors. Subsequent designs, such as that disclosed in U.S. Pat. No. 1,856,204, issued to Affel, et al. on May 24, 1930, described conductor arrangements for providing spare conductors for special services. Nevertheless, the problem of crosstalk remained a major problem for cable makers and users.
As a result, efforts to reduce crosstalk between adjacent conductors or pairs continued. For example, U.S. Pat. No. 1,978,419, issued to Dudley on Oct. 30, 1934, discloses the use of bundled coaxial conductors for supporting bi-directional transmission of signals having similar frequencies while minimizing near end crosstalk. However, coaxial cables tend to be quite large, particularly for large numbers of conductors. Still other techniques were used to achieve improved cable performance such as the use of heavy gauge conductors and special twining or twisting techniques as disclosed in the U.S. Pat. No. 2,014,214, issued to Smith on Sep. 10, 1935.
Spacers or fillers have been used as part of cable configurations for maintaining spacing of conductors. For example, U.S. Pat. No. 2,488,211, issued to Lemon on Nov. 15, 1949, discusses and describes the use of a filler arranged around a central multi-strand conductor for maintaining separation between the central conductor and a surrounding metallic screen in a high frequency cable. Further, U.S. Pat. No. 2,761,893, issued to Morrison on Sep. 4, 1956, discusses the use of a central filler made of fibrous jute in a travelling electrical cable to provide enhanced mechanical balance.
In addition to incorporating various fillers in cables to enhance electrical characteristics, special routing of conductors inside a cable has been used to reduce crosstalk. In particular, U.S. Pat. No. 3,227,801, issued to Demmel on Jul. 4, 1966, describes the technique of using a precise conductor crossing method whereby the distance over which any two conductors are adjacent is minimized.
In addition, various dielectric materials have been used inside cables to enhance electrical characteristics. For example, in U.S. Pat. No. 2,804,494, issued to Fenton on Apr. 8, 1953, conductors of a high frequency transmission line are separated by air, acting as a dielectric, to reduce noise pickup. However, it should be noted that Fenton addresses the problem of external interference and not crosstalk between adjacent conductors within the same cable.
Still other techniques have been employed for maintaining a particular conductor geometry. For example, in U.S. Pat. No. 3,644,659, issued to Campbell on Feb. 22, 1972, resilient filler strings are used as a central core to hold a surrounding layer of conductors against an outer shield. The objective in Campbell's cable is to maintain firm contact between the conductors and the outer shield, even while being flexed, for maintaining a tight tolerance on impedance. Similarly, U.S. Pat. No. 3,678,177, issued to Lawrenson on Jul. 18, 1972, also describes the use of a central filler surrounded by conductor pairs all contained within an outer shield. Therein, Lawrenson discusses the use of dielectric spacers between pairs of conductors rather than the use of tightly twisted pairs. U.S. Pat. No. 4,767,890, issued to Magnan on Aug. 30, 1988, also discusses the use of a central filler, around which conductors are arranged for reducing the "skin effect" across the audio frequency range.
Conventional high frequency telecommunication cable configurations generally employ unshielded twisted pairs (UTP) as the primary cable component. Although many configurations are used in the industry, typical configurations include four twisted pairs and are performance rated by impedance, attenuation and near end crosstalk.
Contemporary commercial building cabling standards facilitate planning and installation of cabling by establishing performance and technical requirements for various system configurations. The most rigid of these standards define specifications for cabling intended to support a broad range of telecommunication services including voice, data, video, and the like.
More recently, the rapid growth in telecommunications, and in particular local area networks, has sparked an increase in demand for high capacity, high performance, high frequency telecommunications cable. To meet this demand, contemporary cable configurations incorporate higher pair counts to make more efficient use of cable space. However, recent industry standards for cables with higher pair counts are more rigorous than standards for lower pair count, such as 4 pair cables. Most significantly, the crosstalk requirement changes from a worst pair requirement to a power sum type requirement which is more far difficult to attain.
Specifically, unlike the traditional Near End Crosstalk (NEXT) standard which identifies and quantifies the worst pair-to-pair combination in the cable, the Power Sum Near End Crosstalk (PSNEXT) standard of a specific pair is the mathematical pair-to-pair near end crosstalk contributions of all other pairs in the cable into that pair. Consequently, PSNEXT determines each twisted pair's resistance to coupled power from all other pairs, summed on a power basis, when all the pairs are simultaneously energized. Such a stringent standard is now used in a network environment where multiple high frequency or high data rate transmissions are employed in a single cable, as can be seen when the cable is used as a backbone for a network, or networks, as part of a structured cabling system.
It is well known in the art that the factors most affecting near end crosstalk are resistive or inductive unbalances, distance between the disturbing and disturbed (or listening) pairs and careful lay length selection. However, even with this knowledge, cable configurations with large twisted pair counts, typically greater than four have been unable to meet the requisite PSNEXT requirements. Alternative approaches such as bundled four pair cables, each with its own jacket with or without an overall jacket, tend to be difficult to manage and install.
One recent approach that provides a cable having a high number of twisted pair count with a superior power sum crosstalk performance is described in U.S. Pat. No. 5,519,173, issued on May 21, 1996, entitled High Speed Telecommunication Cable and assigned to the assignee hereof. This patent application is hereby incorporated herein by reference. The design described in the above-identified patent application provides a high speed, high performance telecommunications cable having a higher twisted pair count while maintaining superior power sum crosstalk performance. However, it has been found that because of the use of filler members about which the twisted pairs of electrical conductors are wrapped, the cable described in this patent application can be relatively expensive. That is, not only is there the expense of the filler members themselves but the cross-sectional size resulting from wrapping all of the twisted pairs of electrical conductors about the filler members of the cable increases the cost of the resultant cable.
Consequently, a relatively inexpensive, high speed, high performance telecommunications cable having a higher twisted pair count while maintaining superior power sum crosstalk performance is highly desirable.